Well, if you and your both know you might have to work late without having advance notice why don't the two of you put your heads together and come up with a backup plan when this happens?
Again. This is making something simple more complicated...
The person that needs the backup plan is the person asking for the "favor".. The other person isn't responsible for the backup plan.
Again.. When she asks..
the answer is a yes or a no.. There is no need to get so deep into this relationship stuff. Not needed...
If Kevin asks her for the favor, then he needs the backup plan. He also should just happily accept her yes or her no or whatever else she chooses to say to him when he asks.....